The present invention relates generally to stents which are implantable or deployable in a vascular or endoluminal location within the body of a patient to maintain the lumen open at that location, and more particularly to improvements in stent coatings.
Stents are expandable prostheses employed to maintain narrow vascular and endoluminal ducts or tracts of the human body open and unoccluded, such as a portion of the lumen of a coronary artery after dilatation of the artery by balloon angioplasty. In the case of an occluded coronary artery, for example, the original blockage is typically attributable to fatty deposits or plaque on the inner lining of the vessel. A different mechanism, however, produces a new blockage after the angioplasty procedure is performed to compress the deposits against the inner lining of the vessel, as by balloon angioplasty, or by removal virtually entirely, using laser angioplasty or rotational cutting. The blood vessel wall is subjected to trauma by any of these procedures, which results in hyperplasia of the neointima, i.e., rapid proliferation of muscle cells in the affected region of the wall, to cause restenosis and re-occlusion of the vessel lumen in a significant percentage of angioplasty patients within a period of from three to six months following the initial procedure.
To avoid this re-occlusion and to maintain the lumen of the vessel open, it is now customary procedure to install a stent at the site in the vessel where the angioplasty was performed. The stent is deployed by radial expansion under pressure exerted by the inflating balloon of a balloon catheter on which the stent is mounted, to engage the inner lining or surface of the vessel wall with sufficient resilience to allow some contraction but also sufficient stiffness to resist to a great degree the natural recoil of the vessel wall that follows its expansion.
The presence of the stent itself in the bloodstream, however, promotes thrombus formation and clotting as blood flows through the vessel. This, too, can result in sufficient blockage of the coronary artery to produce an infarction. Thrombus formation and clotting at the inner lumen of the stent, and fibrosis and restenosis at the site of the vessel wall where the angioplasty was performed and the outer surface of the stent is now engaged, can be significantly reduced by application of appropriate acutely acting drugs in the locality of the stent. Some difficulty is encountered in providing a stent surface which is suitable for retention of the necessary drug(s) to achieve those purposes.
Additionally, it would be desirable to provide a stent with the capability to deliver an effective dose of radiation to tissue at the inner lining of a vessel from which a lesion has been removed or at which a lesion is produced, to further inhibit restenosis at that site. And for purposes of x-ray fluoroscopic viewing of the stent as it is being implanted, as well as examination after the stent delivery system is removed, it is desirable that the stent have a sufficient radiopacity without making its wall so thick that flexibility of the stent and lumen size of the vessel at the implant site are adversely affected.
It is a principal aim of the present invention to provide a stent and method of manufacture thereof which provides a suitable coating on the exposed surfaces of the stent to measurably reduce tissue irritation and, thereby, the traumatic response that produces rapid proliferation of the tissue, as well as to provide improved thrombogenicity, an elongate site for delivery of radiation therapy to irritated tissue, and an improved surface region for carrying an additional biodegradable coating that releases, for example, anti-fibrotic and anti-thrombotic drugs, which also avoids responses tending to initiate a reblockage of the vessel in which the stent is implanted, especially a coronary artery.
Another aim of the invention is to provide a stent with a special coating that resists occlusion of a blood vessel at the implant site attributable to either or both irritation-induced hyperplasia of the intimal and neointimal region of the vessel wall and stent-induced clotting and thrombus formation.
Yet another aim of the invention is to provide a stent which has improved radiopacity for X-ray fluoroscopy viewing without increasing the physical dimensions of the stent.
Still another aim of the invention is to provide a stent with a level of radioactivity sufficient to inhibit cell proliferation at the inner lining of the vessel wall at the implant site, without significant damage to healthy tissue in the vicinity.